Embodiments in accordance with the invention relate to an apparatus for generating a correction signal for linearizing an output signal of a non-linear element, and to a method for generating a correction signal for linearizing an output signal of a non-linear element.
Some embodiments in accordance with the invention relate to a method for linearizing using digital signal processing in accordance with the feedforward principle.
By linearizing amplifiers, for example, undesired signal portions generated within the amplifier may be reduced.
The feedforward method for linearizing amplifiers, for example, has been known for quite some time and has been widely used. FIG. 2 shows a block diagram of a known apparatus 200 for linearizing an amplifier in accordance with the feedforward principle.
An input signal, also referred to as a useful signal below, is applied to the component 10 to be linearized, e.g. an amplifier. Upstream from said component 10, part of the signal is coupled out (as is shown at reference numeral 11), said part serving as a reference signal for useful-signal suppression. Downstream from said component 10, part of the signal is coupled out as well (as is shown at reference numeral 12), said part further being referred to below as an output signal or as a part of the output signal which contains not only the useful signal, but also the portions which are undesired within the component 10, the so-called error signal.
The reference signal is matched, in terms of amplitude (as is shown at reference numeral 13) and phase (as is shown at reference numeral 14), such that at the summation point 16, for example a coupler, said reference signal has the same amplitude and a 180 degrees phase offset as that part of the output signal that was coupled out. To this end, a delay 15, which balances the delay time of the component 10, may also be used.
In the summer 16, also referred to as a combiner, the useful-signal portion is suppressed because of the 180 degrees phase offset. The error signal is maintained since it is contained within only one of the two summed-up signals.
The amplitude (as is shown at reference numeral 22) and the phase (shown above reference numeral 23) of the error signal are matched such that at the summation point 25, for example a directional coupler, said error signal has the same amplitude and a 180 degrees phase offset as the error signal which arrives at the summation point on a direct path from the component 10 via a delayer 21. To this end, an auxiliary amplifier 24 may be used which balances, for example, the losses of the coupler 12 and the summer 25, as well as a delay 21 in the direct path, which balances the delay time of the auxiliary amplifier.
Due to the 180 degrees phase offset, the error signal portion within the output signal is suppressed. The useful signal is maintained since it is contained within only one of the two summed-up paths.
The mode of operation of the feedforward method highly depends on how well the amplitude and the phase of the summed signals may be matched. The better the match between the amplitude and the phase in the summation, the more the undesired signal portion will be suppressed, and the better the entire system will work.
What is also crucial is that no additional spurious signals are generated, for example by non-linear devices within a phase shifter 14, in the reference path of the useful-signal suppression (as is shown at reference numerals 13, 14 and 15). Said spurious signals are not suppressed and appear, in addition to the useful signal, at the output of the feedforward system.
For matching the loops, a phase shifter may be used, for example, by means of which the phase of the signal may be adjusted as accurately as possible. This may be performed, e.g., by analog phase shifters.
One possibility are phase shifters comprising non-linear devices, such as mixers. However, in non-linear devices, harmonics and intermodulations arise which can no longer be removed from the feedforward system.
Another possibility are vector phase shifters, comprised of purely passive devices. Vector phase shifters comprised of purely passive devices create a quadrature signal by means of a passive 90 degrees splitter. The disadvantage is that a 90 degrees splitter has a non-constant group delay time that may be equalized for utilization in a feedforward system.
This is why it is desirable to realize a feedforward system mainly in the digital domain. In this way, it may be possible to implement, e.g., phase shifters in the digital domain. Digital phase shifters may be implemented to be highly accurate and to have a constant group delay time. Thus, the loops may be matched more accurately than in the analog domain, and suppression of the undesired signal portions may be improved accordingly.
Disadvantages of analog phase shifters may be avoided.
To this end, FIG. 3 shows a block diagram of a known apparatus 300 for linearizing an amplifier in accordance with the feedforward principle. The useful signal or input signal is digitized using an analog-to-digital converter 30. After coupling out the reference signal for useful-signal suppression (as is shown at reference numeral 11) in the digital domain, the signal (input signal) is converted back to being analog using a digital-to-analog converter 31, and is supplied to the component 10.
The setup basically corresponds to the apparatus shown in FIG. 2.
The output signal of the component 10 is coupled out in the analog domain (as is shown at reference numeral 12) and is then digitized using an analog-to-digital converter 32. Thus, the summation 16 for useful-signal suppression may be performed in the digital domain, and the components for matching the amplitude 13 and the phase 14, as well as the signal delay 15, may be realized, in the digital domain, as part of digital signal processing.
Matching of error-signal suppression for the amplitude (as is shown at reference numeral 22) and the phase (as is shown at reference numeral 23) is also performed in the digital domain. Prior to the amplification by the auxiliary amplifier 24, the signal may be converted back to being analog using a digital-to-analog converter 33. The summation for error-signal suppression is performed in the analog domain.
For example, the quality of the analog-to-digital converters and of the digital-to-analog converters plays an important part in this context.
In addition, the area 310 of digital signal processing has been marked in FIG. 3.
Well-known examples of this are also set forth in “Building blocks for wideband powerbank for shipboard HF communication systems, Laske, C.; Ulbricht, G.; Heuberger, A., London: IET, 2006, ISBN: 0863416594, ISBN: 9780863416590, pp. 105-109”.
Further well-known examples are set forth in patent documents U.S. Pat. No. 4,389,618, U.S. Pat. No. 4,560,945, U.S. Pat. No. 4,591,6407, U.S. Pat. No. 4,926,134, U.S. Pat. No. 4,926,136, EP 0411180, U.S. Pat. No. 5,157,345, U.S. Pat. No. 5,077,532, U.S. Pat. No. 5,148,117, U.S. Pat. No. 5,300,894, U.S. Pat. No. 544,864, U.S. Pat. No. 5,455,537, U.S. Pat. No. 5,489,875, U.S. Pat. No. 5,789,976, WO 98/04034, U.S. Pat. No. 5,760,646, U.S. Pat. No. 5,862,459, U.S. Pat. No. 5,774,018, U.S. Pat. No. 5,898,338, WO 98/12800, U.S. Pat. No. 5,877,653, U.S. Pat. No. 6,067,448, U.S. Pat. No. 5,994,957, U.S. Pat. No. 6,075,411, U.S. Pat. No. 5,959,500, U.S. Pat. No. 6,078,216, EP 0869606, U.S. Pat. No. 6,531,918, U.S. Pat. No. 6,166,601, EP 0996222, U.S. Pat. No. 6,583,739/WO 01/08293, U.S. Pat. No. 6,392,481, WO 01/41297, U.S. Pat. No. 6,266,517, US 2003/0132802, EP 1124324A1, U.S. Pat. No. 6,275,106, U.S. Pat. No. 6,359,508, EP 1353438, EP 1241781A1. U.S. Pat. No. 6,504,428, U.S. Pat. No. 6,496,064, US 2003/0030490, U.S. Pat. No. 6,407,635, U.S. Pat. No. 6,683,495, EP 1309082, US 2003/0174017, WO03/105337, US 2004/0004516, and US 2003/0184373.
In addition, U.S. Pat. No. 6,188,732 B1 shows a digital feedforward amplifier for utilization in an RF transmitter. The digital feedforward amplifier uses digital signal processing for generating error correction signals, said digital signal processing reducing intermodulation distortions which arise due to the saturation of the main power amplifier.